Closed loop vapor expansion systems normally include, in serial flow relationship, a pump, an evaporator or boiler, a turbine, and a condenser, with a working fluid being circulated therein. A common approach for the evaporator and condenser is to use a tube and shell structure with the working fluid passing through one and another medium passing through the other, in heat exchange relationship therewith. In the case of the condenser, it is common to pass the hot refrigerant vapor from the turbine through the shell while cooling water is passed to the tubes from the cooling tower.
A condenser tube and shell heat exchanger comprises a shell with the plurality of tubes passing therethrough, with the tubes often being constructed with materials dissimilar from the shell. The use of copper in the tubes is often preferred because of its superior heat transfer characteristics, resistance to corrosion, or ease of use in manufacturing. However, because of the differences in the vessel and the tube materials, and their associated expansion coefficients, stress is created in such structures by their exposure to different temperatures and/or temperature difference from the manufacturing reference conditions. That is, at higher temperatures the thermal expansion of copper tubes will be substantially greater than that of steel in the vessel walls, and thereby create thermal stress in the structure.
The problem of thermal stress becomes more serious during periods of emergency shut down when the cooling water is no longer flowing through the condenser, but, because of the continued heat transfer and vaporization within the evaporator, hot refrigerant vapor continues to flow into the condenser, elevating the material temperatures